1. Field of the Invention
This invention relates generally to liquid level detection, and more specifically to optical liquid level detection, wherein quality of the liquid chemical is also determined.
2. Related Art
There are many processes involving liquid chemicals which require consistent and reliable delivery of liquid chemicals with very high purity. In the semiconductor manufacturing industry, for example, reliable delivery of high purity liquids to the process chamber is critical to the overall operation. Catastrophic process events can occur, for example, if liquid levels in canisters or ampoules are not monitored accurately. To make matters worse, many semiconductor manufacturing processes involve very expensive chemicals which have significant health hazards associated with them. These factors require good utilization of the chemical involved along with minimal waste. As a result, there is a need for reliable level sensing of liquid source and intermediate containers in process chemical delivery systems.
Such need for reliable level sensing has been addressed satisfactorily by a number of ways. A simple but reliable float sensor with magnetically activated electromechanical switches can be strategically placed inside the container so as to provide electrical setpoint signals when the liquid level reaches predetermined levels in the container. This concept of reliable level sensing of multiple setpoints can be expanded to any type of reliable level sensor that is internal to the container. Alternatively, with simple implementation and quantitative measure, a weighing scale can be placed under a container in order to monitor amount of liquid in any container. There are also numerous other liquid level sensing devices such as ultrasonic and capacitance. All these liquid level sensing devices are readily available commercially (for example see Zorrich, Bob xe2x80x9cMonitoring Liquid Chemical Levelsxe2x80x9d, Semiconductor International, July 1997, pages 327-336.)
Another means of liquid level sensing are optical methods. For example Fletcher (U.S. Pat. No. 5,046,925) teaches the art of utilizing Bragg optical diffraction phenomenon to sense the presence of liquid at the point of sensing. Although by description this is basically a one-position sensor, theoretically multiple sensors can be cleverly positioned so as to track liquid level within limits. Further, unlike many electrical monitoring based sensors, optical and ultrasonic sensors are less intrusive to chemicals themselves, and are least likely to induce chemical degradation. This is a particularly important point in the semiconductor industry where many process chemicals are very active and hence prone to chemical degradation. Such degradation cannot be tolerated in critical semiconductor manufacturing steps because extremely high purity chemicals are required.
Another, yet more simple, optical approach to level sensing is to use optical transmission and/or reflection phenomenon at the end of light transmission fibers in order to determine if liquid is present or not. As with Bragg diffraction, this technique is again one position level determination. But here, there is the added advantage of more easily scaling up to multiple position monitoring. Unlike the diffraction method, no critical optical alignment is required. Such method has been successfully established on numerous liquid delivery systems sold to the semiconductor industry. In these cases, the fibers are positioned along the side of the container with light propagation horizontal to the liquid level.
As has been noted by one inventor (U.S. Pat. No. 5,652,431) the optical spectral transmission properties of many chemical reagents used in the semiconductor industry is a useful and inexpensive means of evaluating concentration of the chemical reagents in the process delivery line, and as noted in such patents many organometallic reagents for chemical vapor deposition exhibits strong color properties which can be used to determine the concentration in a delivery process line.
It would be an advance in the art of chemical delivery if both level and chemical purity of chemical in a container could be determined using an optical means.
In accordance with the present invention, optical means may be used both to determine liquid level and purity of chemical in a container. This technique takes advantage of properties of some liquid chemicals, such as organometallic precursor chemicals, which tend to change optical properties, such as color, turbidity, and the like, due to impurities and chemical degradation over time. Using the methods and apparatus of the present invention, these features of the chemicals are used to monitor quality of chemical by using an inexpensive fiber optic spectrometer to monitor purity of the chemical.
In accordance with a first aspect of the invention, a method of measuring both concentration and amount of a liquid in a liquid chemical container is presented, the method comprising the steps of:
(a) viewing light emanating from a first optical member that is in visual contact with the liquid, the light emanating from the first optical member having a level indicating quality; and
(b) routing light from a second optical member that is in visual contact with the liquid to means for optical discrimination between liquid chemical based on different optical properties.
Preferred methods are those wherein the first optical member and the second optical member are one and the same member; methods wherein the first optical member and the second optical member are separate members; and methods wherein the viewing step comprises human vision. Also preferred are methods wherein the viewing step comprises machine vision. Further preferred are methods including use of a single light source to generate light which enters the first optical member and the second optical member; methods wherein multiple first optical members are installed in the walls of the liquid chemical containers; methods wherein the means to discriminate is a spectrometer, methods wherein the means to discriminate is human vision; and methods wherein the means to discriminate is a turbidity monitor. Preferably, the optical property is color or turbidity.
A second aspect of the invention is a liquid chemical container apparatus for dispensing liquid chemical, the apparatus allowing measurement of both concentration and amount of a liquid chemical in the liquid chemical container, the apparatus comprising:
(a) a container body having an optical member optically connected (in other words arranged so that the optical member has optical access) to the container body, the optical member having a distal end and a proximal end, the distal end adapted to extend into the liquid chemical contained in the container body;
(b) a light source optically connected to the optical member, the light source able to transmit light through the optical member;
(c) a light receptor optically connected to the optical member, the light receptor able to receive light reflected from the liquid chemical, the light reflected from the liquid chemical being transmitted through the optical member; and
(d) means for optical discrimination between liquid chemical of different optical properties, the means for optical discrimination optically connected to the optical member.
Preferred are those apparatus wherein the optical member is an optical fiber transmission/reflection probe; apparatus wherein the means for optical discrimination is selected from the group consisting of human vision and machine vision; apparatus wherein the machine vision is a spectrometer; and apparatus wherein the optical property is color or turbidity.
A third aspect of the invention is an apparatus for dispensing liquid chemical, the apparatus allowing measurement of both concentration and amount of a liquid chemical in the liquid chemical container, the apparatus comprising:
(a) a container body having a plurality of optical members connected to the container body, the plurality of optical members allowing optical access to liquid chemical contained within the container body, the plurality of optical members positioned on the container body in a fashion so that a user may determine the presence or absence of the liquid chemical at the points where each of the plurality of optical members is positioned; and
(b) means for optical discrimination between liquid chemical of different optical properties, the means for optical discrimination connected to at least one of the plurality of optical members. Preferred are apparatus wherein the means for optical discrimination is selected from a group consisting of human vision and machine vision, particularly wherein the machine vision is a spectrometer.
A fourth aspect of the invention is a liquid chemical container apparatus for dispensing liquid chemical, the apparatus allowing measurement of both concentration and amount of a liquid chemical in the liquid chemical container, the apparatus comprising:
(a) an optical member optically connected to the container body, the optical member having a distal end and a proximal end, the distal end adapted to extend into the container body but not touch liquid chemical contained in the container body;
(b) a light source optically connected to the optical member, the light source able to transmit light through the optical member;
(c) a light receptor optically connected to the optical member, the light receptor able to receive light reflected from the liquid chemical, the light reflected from the liquid chemical being transmitted through the optical member; and
(d) means for optical discrimination between liquid chemical of different optical properties, the means for optical discrimination optically connected to the optical member. Preferred are those apparatus within this aspect of the invention wherein the means for optical discrimination is selected from the group consisting of human vision and machine vision; especially apparatus wherein the machine vision is a spectrometer.
It is known that certain liquid chemicals, such as transition metal-amino and metal-alkoxide complexes, exhibit a difference in color and/or turbidity depending on purity and age of the chemical in storage and can form particulate matter in the presence of moisture or other contaminants. In the present invention, optical transmission/reflection phenomenon are monitored by means of simple fiber optics so that special properties can be monitored in order to evaluate purity of chemical, and such purity analysis can be incorporated into existing fiber optics often used to monitor liquid level in containers.
Many organometallic compounds used in chemical vapor deposition are reactive in nature and are difficult to obtain in pure form commercially. For example, three commercial samples of tetrakisdiethylamino zirconium can have three distinct shades of brown, depending on their age. In chemical vapor deposition processes, it is advantageous if the liquid chemicals are easily degraded at lower process temperatures; however, this leads to the use of less thermally stable compounds. The ultraviolet and visible spectral features of such compounds are typically affected by impurities present in the sample. It is known that some chemicals, for example hafnium t-butoxide, change color over time during storage due to their reactive nature (as disclosed in Material Safety Data sheet from InorgTech dated Oct. 7, 2000.) Furthermore, such compounds react with trace moisture and numerous other sources of oxides to form insoluble metal oxides that become suspended in the liquid causing the liquid to become turbid. This turbidity can also be monitored optically with fiber optics. Thus, in situ monitoring of these chemicals, such as with fiber optics then provides a convenient means of monitoring quality of liquid chemical to be used in critical processes, such as semiconductor manufacturing. At the same time, such optical monitoring can be used to monitor two or more critical parameters in liquid chemical delivery: liquid level in a container, liquid purity in the container, color of liquid chemical in the container, and change of any of these parameters.
Further understanding of the invention will be ascertained upon review of the following brief description of the drawing figures and description of preferred embodiments. The drawing figures are not to scale and are merely meant to be illustrative of the apparatus and methods of the invention.